lchicine binding to tubulin leads to sufficient immobilization of the gand to limit vibrational dissipations of the excited state and hence rmit fluorescence with respectable quantum yields. While binding domains r the A and C rings have been clearly identified, we now provide evidence at the B ring, and its substituents, contributes substantially to this mobilization and profoundly influences the kinetics of binding of the drug. addition, we have shown that the carboxy termini of both subunits of bulin are, by virtue of their large excess of negative charge, major terminants in the ability of the tubulin dimer to polymerize. Selective moval of the C termini markedly enhances microtubule assembly through a -fold lowering of the critical concentration for polymerization.